I have an input file which I'm reading in with the basic myFile >> variable since I know the format and the format will always be correct. The file I'm reading in is formatted as instruction <num> <num> and to make >> work, I'm reading everything in as a string. If I have 3 variables, one to take in each piece of the line, how can I then turn string <1> (for example) into int 1? I know the string's first and last characters are brackets which need to be removed, then I could cast to an int, but I'm new to C++ and would like some insight on the best method of doing this (finding and removing the <>, then casting to int)
use stringstream
#include <string>
#include <sstream>
#include <iostream>
int main() {
std::string str = "<1>";
int value;
std::stringstream ss(str);
char c;
ss >> c >> value >> c;
std::cout << value;
}
First to get the middle character out you can just do char myChar = inputString.at(1);. Then you can do int myInt = (int)myChar;
Even if you remove the <> characters, your still importing the file content into a string using >> so you still need to cast it to an int. If you have only 1 value, you can follow what Nicholas Callahan wrote in the previous answer, but if you have multiple characters you want to read as int, you dont have a choice but to cast.
You can also resort to sscanf.
#include <cstdio>
#include <iostream>
#include <string>
int main()
{
std::string str = "<1234>";
int value;
sscanf(str.c_str(), "<%d>", &value);
std::cout << value << std::endl;
}
Related
I need to convert a std::string to a const char*.
To do so, I used the c_str() method on the string, as in the following code :
#include <iostream>
#include <string>
using namespace std;
int main(void) {
string testStr;
cin >> testStr;
const char* testStrConst = testStr.c_str();
cout << testStrConst << endl;
return 0;
}
If I type "Hey hello" in the terminal, when this code is running, the output is only "Hey".
Why is the second word ignored?
Because it was never a part of the std::string in the first place.
The >> operator reads only a single, whitespace delimited word.
Use std::getline() instead of >> to read the entire line of text entered on standard input.
string testStr;
getline(cin, testStr);
I have a string (std::string) which contains a MAC address in C++, e.g.:
10:10:0F:A0:01:00
I need to convert it to an array of bytes (unsigned char*).
The bytes have to be written from left to right. Does anybody have a function or efficient algorithm for this?
Sorry for necroposting, but just to help others who may still be searching for the answer, there is a standard C way that still can be used in C++ without any reinvention of the wheel. Just man ether_aton or click here.
This'll work. You've tagged this as C++ so I've scrupulously avoided the shorter solution that's possible using the sscanf C method. using namespace std is used here only to shorten the quoted code.
#include <iostream>
#include <sstream>
main() {
unsigned char octets[6];
unsigned int value;
char ignore;
using namespace std;
istringstream iss("10:10:0F:A0:01:00",istringstream::in);
iss >> hex;
for(int i=0;i<5;i++) {
iss >> value >> ignore;
octets[i]=value;
}
iss >> value;
octets[5]=value;
// validate
for(int i=0;i<sizeof(octets)/sizeof(octets[0]);i++)
cout << hex << static_cast<unsigned int>(octets[i]) << " ";
cout << endl;
}
i want to extract number string values of a char array. Actually I want to extract numbers embeded in file names for some file management. For example if there is a file name as file21 then i want the decimal number 21 from this file name.
How can i extract these values?
I tried the following but it results in an unexpected value. I think it is as a result of the implicit typecasting from the char to int while doing the arthimetic operation.
char * fname;
cout<<"enter file name";
cin>>fname;
int filenum=fname[4]%10+fname[5];
cout<<"file number is"<<filenum;
NOTE:
The filenamse are strictly in the format fileXX, XX being numbers between 01 and 99
You need to subtract '0' to get the decimal value of a digit character:
int filenum=(fname[4]-'0')*10+(fname[5]-'0');
Better yet, you should use atoi:
int filenum = atoi(fname+4);
You're getting undefined behavior because you're never allocating memory for the char* you read into:
char * fname = new char[16]; //should be enough for your filename format
or better yet
char fname[16];
Also, what do you expect:
fname[4]%10+fname[5];
to do? Magically concatenate the numbers?
First, you convert the first char to an int, multiply it by 10, convert the second char to an int and add to the first one. A simple google search for char to int would get you there.
How can i extract these values?
There are an infinite number of ways. One way is to use std::istringstream:
#include <string>
#include <sstream>
#include <iostream>
int main () {
std::string fname;
std::cout << "Enter file name: ";
std::getline(std::cin, fname);
int filenum;
std::istringstream stream(fname.substr(4,2));
if(stream >> filenum)
std::cout << "file number is " << filenum << "\n";
else
std::cout << "Merde\n";
}
Here is the simplest code:
#include <iostream>
#include <sstream>
#include <string>
using namespace std;
int main()
{
int filenum;
string fname;
cout<<"enter file name";
cin>>fname;
string str2 = fname.substr(4,2);
istringstream(str2) >> filenum;
cout<<"file number is"<<filenum;
return 0;
}
If your input is that much defined, the simplest solution is scanf:
int main()
{
int theNumber = 0;
scanf("file%d.txt", &theNumber);
printf("your number is %d", theNumber);
}
Check it out in action, reading from char* instead of stdio: http://codepad.org/JFqS70yI
scanf (and sscanf) also throws in checking for proper input format: returns number of fields read successfully. In this case if return value is any other than 1, the input was wrong.
I have an integer 1 and i want to display it as a character '1' in C++. So far I have only managed to convert it from say integer 65 to character 'A'.
How do you stop this ?
int theDigit = 1;
char ch = theDigit+'0';
This works because it's guaranteed1 that the sequence of characters '0'...'9' is contiguous, so if you add your number to '0' you get the corresponding character. Obviously this works only for single digits (if theDigit is e.g. 20 you'll get an unrelated character), if you need to convert to a string a whole number you'll need snprintf (in C) or string streams (in C++).
C++11, [lex.charset] ΒΆ3:
In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous.
By the way, I suppose that they didn't mandate contiguity also in the alphabetical characters just because of EBCDIC.
Use the stringstream.
int blah = 356;
stringstream ss;
string text;
ss << blah;
ss >> text;
Now text contains "356"(without quotes). Make sure to include the header files and use the namespace if you are going to copy my code:
#include <sstream> //For stringstream
#include <string>
using namespace std;
#include <stdio.h>
#include <stdlib.h>
int i = 3;
char buffer [25];
itoa (i, buffer, 10);
printf ("Integer: %s\n",buffer);
Integer: 3
You did just ask about printing an integer, so the really simple c++ answer is:
#include <iostream>
int main()
{
int value = 1;
std::cout << value << endl;
return 0;
}
i have a little problem on writing the string into a file,
How can i write the string into the file and able to view it as ascii text?
because i am able to do that when i set the default value for str but not when i enter a str data
Thanks.
#include <iostream>
#include <fstream>
#include <cstring>
using namespace std;
int main()
{
fstream out("G://Test.txt");
if(!out) {
cout << "Cannot open output file.\n";
return 1;
}
char str[200];
cout << "Enter Customers data seperate by tab\n";
cin >> str;
cin.ignore();
out.write(str, strlen(str));
out.seekp(0 ,ios::end);
out.close();
return 0;
}
Please use std::string:
#include <string>
std::string str;
std::getline(cin, str);
cout << str;
I'm not sure what the exact problem in your case was, but >> only reads up to the first separator (which is whitespace); getline will read the entire line.
Just note that >> operator will read 1 word.
std::string word;
std::cin >> word; // reads one space seporated word.
// Ignores any initial space. Then read
// into 'word' all character upto (but not including)
// the first space character (the space is gone.
// Note. Space => White Space (' ', '\t', '\v' etc...)
You're working at the wrong level of abstraction. Also, there is no need to seekp to the end of the file before closing the file.
You want to read a string and write a string. As Pavel Minaev has said, this is directly supported via std::string and std::fstream:
#include <iostream>
#include <fstream>
#include <string>
int main()
{
std::ofstream out("G:\\Test.txt");
if(!out) {
std::cout << "Cannot open output file.\n";
return 1;
}
std::cout << "Enter Customer's data seperated by tab\n";
std::string buffer;
std::getline(std::cin, buffer);
out << buffer;
return 0;
}
If you want to write C, use C. Otherwise, take advantage of the language you're using.
I can't believe no one found the problem. The problem was that you were using strlen on a string that wasn't terminated with a null character. strlen will keep iterating until it finds a zero-byte, and an incorrect string length might be returned (or the program might crash - it's Undefined Behavior, who knows?).
The answer is to zero-initialize your string:
char str[200] = {0};
Supplying your own string as the value of str works because those in-memory strings are null-terminated.